Be careful with old versions!
These versions are displayed for reference and testing purposes. You should always use the latest version of an add-on.
But without this one line fix, the addon just does not load due to an unresolved import. If the but in Firefox is fixed, the addon should work again.
The pdf is reloaded in the plugin immediately, even if the plugin preference is set to click to play, because it is assumed that the user wants to start the addon if s/he chooses the context menu item (or toolbar button).